The XtCallCallbacks function calls each procedure that is
registered in the specified widget's callback list.

If callbacks is not NULL,
XtCallCallbackList calls each of the callback procedures in the list,
passing client data and call_data. The callbacks parameter must specify
the contents of a widget or object resource declared with representation
type XtRCallback.

The XtHasCallbacks function first checks to see if
the widget has a callback list identified by callback_name. If the callback
list does not exist, XtHasCallbacks returns XtCallbackNoList. If the
callback list exists but is empty, it returns XtCallbackHasNone. If the
callback list exists and has at least one callback registered, it returns
XtCallbackHasSome.